Photographic paper having reduced processing curl



Feb. 24, 1959 w. H. GRIGGS ETAL 2,875,055

PHOTOGRAPHIC PAPER HAVING REDUCED PROCESSING CURL Filed Oct. 18, 1956 4 EMULSION GELAT/N- BARYTA 3 9 POLYSTYRENE-GELAT/N I PAPER William II. Griggs FranloA.Builer 1 paper stock which other side.

United States Patent PHOTOGRAPHIC PAPER HAVING REDUCED PROCESSING CURL Application October is, 1956, Serial No. 616,597

3 Claims. or. 96-85) This invention concerns a method of treating photographic paper to eliminate or reduce the processing curl.

Early in the history of photographic paper manufacture, a slow rate of drying was used following coating with photographic emulsion. The curling characteristics ofthe paper thus prepared were not too unfavorable, particularly when the paper was stored in a place where there was not a greatdeal of variation in the relative humidity of the air in the place of storage. In recent years, however, the tendency has been to speed up the rate of drying of the emulsion-coated paper, which has involved the use of elevated temperatures in those drying operations. 9

The cellulose fiber in the paper base of photographic paper and the gelatin in the emulsion layer thereof-are both hygroscopic. They expand as they absorb water vapor at-high humidity and contract asthey desorb water vapor at'low humidity! The rates of expansion and contraction of these two materials are not necessarilythe same under a given setof conditions, so that the stresses and strains set up in photographic paper often result in curl. This is especiallytrueinthe case of lightweight paper strokes such as'those having a weight of 8-15 poundsper 1000 square feet. When such paperis coated with an emulsion, often the stress in the emulsion layer is sufliciently great to resultinface curl, or in other words, the curling of the paper toward the emulsioncoated side. p When wet photographic emulsion is applied to dry paper, some of the moisture from the emulsionpenetrates the paper resulting in expansion of the paper base, while loss of water bythe emulsion causes its contraction. Thus, the expansion of the paper and-the contraction of the emulsion at that time sets up a balance of forcessomewhat different from that obtained in the paper andtheemulsion immediately prior to the absorption of watenb y the attern To avoid internal crackling of certain emulsions-the photographic emulsion is coated on the wire side of provides a denser surface than the As a result, the paper may curl toward the back (non-coated) side during processing when the processingsolutions penetrate the paper and emulsion. The curl actually starts out as a face curl (toward the coated side) immediately upon immersion in the developer and then reverses to a back curl (toward the uncoated side) after about half the development time is up. While at the time of development, the curl may be extreme, it may become even more pronounced during the balance of the processing cycle especially when a water rinse is used in place of short-stop and when the wash water, after fixing, is on the alkaline side. In addition, the process curl may tend to become more noticeable as the sensitized paper ages, usually reaching the worst point at about six months age.

In the co-pending patent application, Serial No. 467,631, Patent No. 2,784,088, in the name of William H. Griggs, filed November 8, 1954, is disclosed a method ject is to provide a photographic paper 2,875,055 Patented Feb. 24, 1959 .2 I of reducing processing curl in photographic paper by coating a light size coat of a non-continuous film of polystyrene under the .baryta on the wire-side coated paper. In the operation of this process, 254 grams per square meter of a polystyrene resin latex is applied to the paper surface, and the surface heat-treated at a temperature of 275-320 F. for 5-l0 seconds in order to fuse the polystyrene. to form a discontinuous coating. However, this requires that a heating step be applied following the application of the polystyrenecoating. Inasmuch as each operation which must be carried out in the production of photographic paper requires additional expense, there has been a need for a method of preparing photographic paper having reduced processing curl in which the size could beapplied at the size press on the paper machine as the paper is being made.

i We have found unexpectedly that photographic paper can be prepared having reduced processing curl by immersing the stock in a polystyrene resin latex and then passing it through the paper machine press rolls to force the latex into the stock and remove the excess from the surface. This results in, reducing the bulking by almost 50 percent. Bulking is a measure of the increase in thickness of a paper due to wetting. For example: When a photographic paper base is baryta coated it is wet. This causes a swellingof the stock which in turn gives a rougher surface. The stock therefore has to be supercalendered to reduce the caliper and improve the surface. The same action occurs during sensitizing and processing. Therefore, a stock with less bulk would be thinner, more compact and stiff, and tendto have a smoother surface. difference between paper surface-coated and paper sizepress impregnated:

One object of our invention is to provide a photographic paper with reduced process curl. Another obcoated by immersing the paper in a resin latex and then passing it through press rolls to force the latex into the stock and remove. the excess fromthe surface. An additional object is to provide a method for treating photographic paper, so that the paperwill withstand wide variations in relative humidity without any appreciable curl. A further object is to provide a method of reducing curl in photographic paper bylimpregnating with resin latex which, omits a separate drying step and which permits the resin latex to be partially fused in the regular paper drying operation. p

The above objects are obtained by applying a polystyrene resin dispersion as a size press coating. The paper is first immersed in thelatex and then passed through press rolls to force the latex. into. the stock and remove theexcess from sensitized, shows bad curl after the sensitized paper is four to six months old. In addition, the Fourdrinier sheet normally shows bad processing curl (age) away from the emulsion no matter what side (face or wire) the emulsion is on. A cylinder sheet or a pressure Fourdrinier sheet usually exhibits a bad curl only when the emulsion is coated on the wire side.

The. following table illustrates the the surface, This method is particularly applicable to a Fourdrinier sheet which, While not'exhibiting' poor processing curl when freshly- We have found that a latex gel (70-30) combination is preferable, and a solids content in the resin dispersion of about 20 percent. The range may be 320 percent solid polystyrene and -1-1-0 percent solid gelatin. In our preferred embodiment we use 14 percent polystyrene and 6 percent gel or a total of 20 percent solids The polystyrene retained by the paper is with the range of 0.7-4.5% based on the dry weight of the impregnated paper. The gelatin range is 1.5- on the same basis.-

The accompanying drawing illustrates a paper section showing the surface layers applied according to our invention. The paper support 1 has a polystyrene-gelatin subbing 2 which is applied to the paper support 1 while the paper is wet and as it comes off the paper-making machine. The paper is then dried at which time the temperature of the paper surface is maintained at a temperature of about 200 F. After drying the paper is then coated with a baryta-gelatin 3 coating composition and dried. On top of the gelatin-baryta layer 3 is coated the light-sensitive silver halide photographic emulsion 4.

The following examples illustrate but are not intended to limit my invention:

Example 1 Weeks aging.- The processing curl is measured in the rinse water between the developer and fix portions of the processing cycle as shown below.

Fresh, Aged, cm. cm.

Regular gel size comparison; 5. 8 1l. 5 Polystyrene-gel sized 4. O 3. 9

The curl values are in centimeters and are reported as the decrease in the chord of the arc formed during processing 'of a l0-cm. diameter disk. A completely flat disk would have no arc and therefore would have a value of zero. A disk curling sufliciently for the edges to meet would have a value of 10. A value over 10 would designate wrapping around to form a tube. A sign designates curl away from the emulsion-coated side.

The resin used was supplied as a 30 percent solids dispersion in water. The gelatin was added at a concentration so as to bring the total solids content of the coating composition to percent. A value of 20 percent is optimum with a range of 5-25 percent being usable.

Example 2 Paper prepared as in Example l'was baryta-coated and sensitized on the wire rather than the face side.

Processing curl results on fresh and aged paper showed:

' Example 3 A roll of 27-1b. per 1000 sq. ft. of photographic paper base was tub-sized at the size press with a composition consisting of parts polystyrene resin and 30 parts gelatin on a solids basis but with 0.5 percent potassium hydroxide (based upon polystyrene weight) added. The addition of the KOH destroyed the film of wetting agent which sometimes forms on the surface of latex coatings and causes scummy mottle in paper sensitized with high contrast emulsions.

In the above examples the paper, following coating and impregnation by the size press rolls, was passed through paper machine dryers at a surface temperature of about 200 P. which sufliciently fused the coating.

We have found that the optimum of 60-80 parts resin and 40-20 parts gel are usable in our invention. Above parts resin it is difficult to get the baryta to adhere to the paper. Below 60 parts, the effect of the gel overcomes the efiect of the resin.

An advantageof this method is that the impregnated paper can be baryta coated on either side. In addition, there is an economic advantage in coating at the paper machine size press where paper is normally sized.

We claim: 7

l. A process for providing a photographic paper having improved processing curl comprising .tub-sizing the Wet paper immediately after forming on the paper-making machine and while still wet with a polystyrene-gelatin dispersion, passing the wet paper under a size press roll to force the dispersion through the stock and to remove the excess from the surface, drying the paper at a surface temperature of about 200 F., coating with a baryta adhesive coating composition, drying, and coating with a light-sensitive silver halide emulsion over the dry baryta layer.

2. A process for providing a photographic paper having improved processing curl, comprising tub sizing the Wet paper immediately after forming on the paper-making machine and while-still wet with a polystyrene-gelatin dispersion, passing the wet paper through press rolls to force the dispersion into the stock to remove the excess from the surface so that 0.7-4.5% polystyrene based on the dry coated weight of the paper is retained in the paper, drying the paper at a surface temperature of about 200 F., coating the impregnated paper with a baryta adhesive coating composition, drying, and coating with a light-sensitive silver halide emulsion over the dried baryta layer.

3. A photographic paper obtained by the process described in claim 1.

'References Cited in the file of this patent UNITED STATES PATENTS 2,756,150 Griggs'et a1. July 24, 1956 2,784,088 Griggs Mar. '5, 1957 FOREIGN PATENTS 692,105 Great Britain May 27, 1953 

1. A PROCESS FOR PROVIDING A PHOTOGRAPHIC PAPER HAVING IMPROVED PROCESSING CURL COMPRISING TUB-SIZING THE WET PAPER IMMEDIATELY AFTER FORMING ON THE PAPER-MAKDISPERSION, PASSING THE PAPER UNDER A SIZE PRESS ROLL TO FORCE THE DISPERSION THROUGH THE STOCK AND TO REMOVE THE EXCESS FROM THE SURFACE, DRYING THE PAPER AT A SURFACE TEMPERATURE OF ABOUT 200* F. COATING WITH A BARYTA ADHESIVE COATING COMPOSITION, DRYING, AND COATING WITH A LIGHT-SENSITIVE SILVER HALIDE EMULSION OVER THE DRY BARYTA LAYER
 3. A PHOTOGRAPHIC PAPER OBTAINED BY THE PROCESS DESCRIBED IN CLAIM
 1. 